Trying to diagnose hung connection in UC download.

Observed in 1.580.1 on credentials-plugin (which you would think would be tiny and fast to get):
"Update center installer thread [#1]" #70 daemon prio=5 os_prio=0 tid=0x00007f5634815800 nid=0x64a8 runnable [0x00007f567cfe4000]
   java.lang.Thread.State: RUNNABLE
	at java.net.SocketInputStream.socketRead0(Native Method)
	at java.net.SocketInputStream.read(SocketInputStream.java:150)
	at java.net.SocketInputStream.read(SocketInputStream.java:121)
	at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
	at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
	- locked <0x00000007aa3ec760> (a java.io.BufferedInputStream)
	at sun.net.www.MeteredStream.read(MeteredStream.java:134)
	- locked <0x00000007aa3eedd8> (a sun.net.www.MeteredStream)
	at java.io.FilterInputStream.read(FilterInputStream.java:133)
	at sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:3322)
	at sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:3315)
	at org.apache.commons.io.input.ProxyInputStream.read(ProxyInputStream.java:77)
	at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:766)
	at hudson.model.UpdateCenter$DownloadJob._run(UpdateCenter.java:1138)
	at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1298)
	at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:1116)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at hudson.remoting.AtmostOneThreadExecutor$Worker.run(AtmostOneThreadExecutor.java:104)
	at java.lang.Thread.run(Thread.java:745)